diff --git a/ChangeLog b/ChangeLog
index 42e53bd98b..14d48cafac 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Mon Dec  9 11:14:26 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/date/date_parse.c (parse_time): unset case-insensitive flag
+	  for [:alpha:], which already implies both cases, to get rid of
+	  backtrack explosion.  [ruby-core:58876] [Bug #9221]
+
 Mon Dec  9 08:40:40 2013  Eric Hodel  <drbrain@segment7.net>
 
 	* lib/rubygems:  Update to RubyGems master bf37240.  Fixes useless
diff --git a/ext/date/date_parse.c b/ext/date/date_parse.c
index 29dbb239bb..3003b45239 100644
--- a/ext/date/date_parse.c
+++ b/ext/date/date_parse.c
@@ -765,9 +765,9 @@ parse_time(VALUE str, VALUE hash)
 		   "("
 		     "(?:gmt|utc?)?[-+]\\d+(?:[,.:]\\d+(?::\\d+)?)?"
 		   "|"
-		     "[[:alpha:].\\s]+(?:standard|daylight)\\stime\\b"
+		     "(?-i:[[:alpha:].\\s]+)(?:standard|daylight)\\stime\\b"
 		   "|"
-		     "[[:alpha:]]+(?:\\sdst)?\\b"
+		     "(?-i:[[:alpha:]]+)(?:\\sdst)?\\b"
 		   ")"
 		")?";
     static VALUE pat = Qnil;